Package: firefox-esr Version: 52.2.0esr-1~deb9u1 I have confirmed that issue is still happening in Debian 9. I initially observed the issue in my Debian 7 machines too. The switch from iceweasel to firefox-esr seems to be when I really started seeing issues with the videos on youtube.
When watching youtube videos, the videos performance are poor, even in low resolution. Fullscreen videos perform worse or don't play at all. This is happening on all machines. I can provide more details if needed. Configuring firefox in about:config media.web.enabled I set this value to false. The problem goes away. I think problem lies with firefox and webm videos. Chromium does not have this issue with videos on youtube.
